The aim of the present work was to assess the prevalence of the endemic goiter in Asturias southwestern area (46,000 inhabitants; 6,638 in school age), after 10 years of iodized salt introduction in diet. 317 schooling children were randomly chosen (stratified by age, sex and geographic area). We evaluated thyroid gland size and urinary iodine excretion (microgram of iodine per gram of creatinine = I/C). Previously, studies of infantile population suggested a 63% of prevalence. Our results indicate a decrease to 16.4% of endemic goiter prevalence. Nevertheless, the success of iodine prophylaxis was heterogeneous and not good in some case: point out Ibias population with a prevalence of 48.5% (2,000 inhabitants and 353 in schooling). Remainder population was prevalences between 6.9% and 19.8%. The mean of urinary iodine excretion increased to I/C = 106 (previously I/C = 53). The smallest index I/C become to Ibias population = 78. (p < 0.01 and p < 0.05 with regard to Cangas del Narcea and Tineo population respectively). Only 2 of the 52 cases of goiter were hypothyroidism (but because of autoimmune thyroiditis). We conclude that iodine prophylaxis with salt was effective generally, in order to diminish the endemic goiter in the area.